Hitherto, there have been known various inkjet recording systems including on-demand ejection systems and continuous ejection systems, as described, for example, in Takeshi Agui, et al., Real Color Hard Copy, Sangyo Tosho Co., Ltd. (1993), Shin Ohno, Non-impact Printing—Technologies and Materials—, CMC Publishing Co., Ltd. (1986), and Takeshi Amari, Inkjet Printers—Technologies and Materials—, published by CMC Publishing Co., Ltd. (1998). Further, the continuous type includes electrostatic systems (for example, Sweet type and Hertz type), and the on-demand type includes a piezoelectric system, a shear mode piezoelectric system, a thermal inkjet system and a recording system called an electrostatic acceleration type. As inks to be used in these inkjet recording systems, aqueous inks that are free from ink clogging in an ink discharge section or an ink supply passage, excellent in discharge stability and good in color image quality, for example, color and gloss are ordinarily used.
As the on-demand type inkjet system using an electrostatic force, systems called electrostatic acceleration type inkjet or slit jet as described, for example, in Susumu Ichinose and Yuji Ohba, Denshi Tsushin Gakkai Rombunnshi, Vol. J66-C (No. 1), p.47 (1983), Tadayoshi Ohno and Mamoru Mizuguchi, Gazo Denshi Gakkaishi, Vol. 10 (No. 3), p.157 (1981) are known. Specific embodiments thereof are disclosed, for example, in JP-A-56-170 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-56-4467 and JP-A-57-151374. In these systems, an ink is supplied from an ink tank into a slit-like ink chamber having a plurality of electrodes disposed inside a slit-like ink-holding section, and a high voltage is selectively applied to these electrodes, thereby ejecting the ink in the vicinity of the electrode to recording paper closely positioned to the slit-like head.
An electrostatic system of a concentration discharge type without using the slit-like recording head is described in JP-A-10-138493. In this system, a plurality of individual electrodes for allowing an electrostatic force to act on a colorant component in ink are constituted of a control electrode substrate composed of an insulating substrate having a through-hole formed therein and a control electrode formed corresponding to the through-hole and a convex ink guide arranged in the substantially center position of the through-hole, the ink is carried on the surface of a convex ink guide to an ink droplet ejecting position by a surface tension, and a prescribed voltage is applied to the control electrode to eject ink droplet to a recording medium, thereby conducting recording.
As inks to be used for these various inkjet recording systems, inks prepared by dissolving various water-soluble dyes in water or a solvent composed of water and a water-soluble organic solvent and optionally adding various additives thereto (hereinafter referred to as “aqueous dye ink”) are mainly employed. However, in the case where printing is practically carried out using the aqueous dye ink, many drawbacks are encountered in that the ink blurs on recording paper depending on the kind of paper, whereby high-quality print can not be obtained, in that a formed recorded image is poor in water resistance and light fastness, in that drying of ink on recording paper is so slow that streaks occur, and in that a recorded image is deteriorated due to color mixing (color turbidity or color unevenness occurred on the interface when dots having different colors are printed adjacent to each other).
For improving the water resistance and light fastness of recorded image that are the problems of aqueous dye ink as described above, there have been made various proposals to apply pigment based ink comprising fine particles of a pigment dispersed in an aqueous solvent or a non-aqueous solvent to the inkjet recording system. For example, inks for inkjet printer comprising a pigment dispersed in a solvent mainly composed of water are proposed in JP-A-2-255875, JP-A-3-76767, JP-A-3-76768, JP-A-56-147871 and JP-A-56-147868. However, there is a problem in that since the pigment is insoluble in the medium, dispersion stability of the ink is ordinarily poor to likely cause clogging in a nozzle section.
On the other hand, ink comprising a pigment dispersed in a non-polar insulating solvent (hereinafter referred to as “oil based pigment ink”) has advantages in that it is less in blur due to good absorption on paper and in that a recorded image is good in water resistance. For example, JP-A-57-10660 proposes oil based pigment ink in which a pigment is pulverized with an alcoholamide dispersant, and JP-A-57-10661 proposes oil based pigment ink in which a pigment is pulverized with a sorbitan based dispersant. However, such inks still have a problem in that clogging of ink in a nozzle section is liable to occur, because it is not sufficient to uniformly disperse the pigment particles in the state of fine particles in the non-polar insulating solvent and the dispersion stability thereof is inferior. In addition, there is a large defect in that the ink is poor in scratch resistance because the pigment itself does not have a fixing ability on recording paper.
For resolving these problems, there are proposed resin dissolution type oil based inks using a resin soluble in a non-polar insulating solvent as a fixing agent and a pigment dispersant. For example, JP-A-3-234772 proposes a terpene phenol based resin as the above-described resin. However, the proposed resin is still insufficient with respect to dispersion stability of pigment and is questionable in reliability as ink. Moreover, since the resin is dissolved in the non-polar solvent, the resin does not remain in an amount sufficient for completely fixing the pigment on recording paper, so that water resistance and scratch resistance are not sufficient.
Thus, for obtaining high-level scratch resistance, it is proposed to coat pigment particles with a resin insoluble or semi-soluble in the non-polar insulating solvent. For example, JP-A-4-25574 proposes oil based ink comprising a pigment coated with a resin by microencapsulation, etc. However, since it is difficult to uniformly disperse the pigment-included resin particles in the state of fine particle and dispersion stability thereof is not sufficient, there is a problem in reliability as ink.
In addition, in recent years, high image quality with photographic image quality is attained by ordinary inkjet printers using the aqueous dye ink. With respect to the pigment ink, for increasing color forming property and transparency, it is required to make pigment fine as far as possible and to keep the dispersion state thereof stably.
However, in contrast, when the pigment is made finer, crushing of pigment primary particles occurs simultaneously with pulverization of the pigment. Further, since cohesive energy simultaneously becomes large due to increase of surface energy, re-coagulation of the pigment particles is apt to occur. As a result, a problem occurs in that storage stability of the pulverized pigment dispersion is damaged. As described above, with respect to the pigment dispersion used in oil based pigment ink for inkjet printer, pulverization at a higher level is required. However, high-level techniques are required for dispersing pigment in the state of fine particle, and it is very difficult to increase the dispersion stability thereof. Therefore, it is the actual situation that oil based pigment ink capable of meeting the above-described requirements is hardly available.
Moreover, in the case where such oil based pigment ink is used for an electrostatic inkjet printer or as an electrophotographic liquid developer, control of charge polarity and stability of the charge with the lapse of time are required. However, since it is very difficult to control the polarity on the pigment surface, it is the actual situation that oil based pigment ink capable of meeting the above-described requirements is hardly available. The electrophotographic liquid developer using a non-aqueous solvent is ordinarily produced by pulverizing a mixture comprising an aliphatic hydrocarbon solvent, a coloring agent, a fixing resin and a dispersant, and optionally added various additives in a ball mill or an attritor, etc. Various methods for the production thereof have been proposed. On the other hand, JP-A-63-174070 discloses a colored liquid developer comprising as a coloring agent, a polymer latex dyed with a dye, wherein the polymer latex is obtained by polymerization of styrene or an acrylic monomer in a non-aqueous solvent. However, the method of using a dye as the coloring agent has defects in that preparation of a black liquid developer is difficult, in that an image density is low because of the dye system, and in that light fading occurs. As an example using a pigment as the coloring agent, JP-B-62-3859 (the term “JP-B” as used herein means an “examined Japanese patent publication”) proposes an electrophotographic liquid developer containing a pigment and as a fixing resin, a resin obtained by reacting a natural resin-modified thermosetting resin with a long chain alkyl group-containing monomer. Although the effect for improving dispersion stability of coloring agent is found, the electrophotographic liquid developer is still insufficient in the dispersion stability. Thus, with respect to the electrophotographic liquid developer using a pigment as the coloring agent, sufficient dispersion stability as well as scratch resistance have been desired. In addition, since pigments are different in a charge polarity depending on the kinds thereof, it has been desired to make the charge polarity of pigment particle clear and to prevent change of the charge polarity with the lapse of time.